Araqchi met Thursday with European Union foreign policy chief Catherine Ashton to discuss Iran's controversial nuclear enrichment program. He said talks were "highly fruitful" but felt there were issues in the way of a formal resolution.
"There are certain serious differences which were discussed in this session, [yet] we haven't reached the stage to discuss the text [of a deal] seriously," he was quoted by Iran's semiofficial Fars News Agency as saying.
The White House said it wanted to put a hold on some economic sanctions targeting Iran in exchange for concessions from Iran. Araqchi stuck to the line, however, that its uranium enrichment program wasn't up for debate.
"Enrichment is our red line and [it] will not be stopped at all," he said.
Iran says it wants to enrich uranium to 20 percent purity for medical research. Western adversaries say that level of purity is indicative of Iran's interest in a nuclear weapon.
U.S. State Department spokeswoman Jen Psaki said during her Wednesday press briefing the U.S. government was questioning Iran's right to enrich uranium.
"Enrichment is a big factor here, and the ongoing enrichment by the Iranians is of great concern," she said.